Volcanoes

The possibility of a large volcanic eruption provides arguably the largest uncertainty concerning the evolution of the climate system. Together with the varying solar activity and astronomical variations, strong volcanic eruptions are a major driving factor of natural climate variability. A sound knowledge of the impact of volcanic eruptions is therefore essential to understand both historical and possible future climate evolution.

Strong volcanic eruptions, which inject sulfur rich gases into the stratosphere, affect global and regional climate on short and long timescales. From sulfuric gases, sulfate aerosols form and reflect sunlight back into space. Thus, the surface of the Earth is cooling considerably. At the same time, sulfate aerosols trap the energy of thermal radiation coming from the Earth's surface. This leads to a pronounced warming of the stratosphere. These two effects lead to changes in the atmospheric and ocean circulation and alter the hydrological and carbon cycle.

Schematic overview of the climate effects after a very large volcanic eruption (from Timmreck, 2012).

The impact of volcanic aerosol on climate and on multi-year seasonal and decadal climate predictability is still largely unexploited. Decadal climate predictions are still uncertain, also due to the unpredictable volcanic activity during the prediction period. This is addressed in the ALARM project.

Volcanic eruptions are natural experiments revealing the sensitivity of the climate system to perturbations. Their simulation with coupled stratosphere-troposphere-ocean models provides more insight in the origin of climate sensitivity of our models. However, the simulation of volcanically-induced climate variability remains a challenging task for climate models. CMIP5 analyses have shown that climate models’ capability to accurately and robustly simulate observed and reconstructed volcanically-forced climate behavior remains poor (Zanchettin et al., 2016). It is so far not clear whether the large inter-model spread in the simulated post-volcanic climate response mostly due to uncertainties in the imposed volcanic forcing or to an insufficient representation of climate processes. To discriminate the individual uncertainty factors it is useful to develop standardized experiments/model activities that systematically address specific uncertainty factors. We have followed this approach in our group over the last years and focus separately on the two major aspects of uncertainties in the post-volcanic model response partly within two large international model intercomparison projects.

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The SSiRC Interactive Stratospheric Aerosol Model Intercomparison project (ISA-MIP) covers the uncertainties in the pathway from the eruption source to the volcanic radiative forcing.

The CMIP6 Model Intercomparison Project on the climate response to Volcanic forcing (VolMIP) addresses the pathway from the forcing to climate response and the feedback, studying the uncertainties in the post-volcanic climate response to a well-defined volcanic forcing (Zanchettin et al., 2016).